gpio/xilinx: Use of_mm_gpiochip_remove

Use the newly created of_mm_gpiochip_remove function for cleaning up
of_mm_gpiochip_add

Suggested-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>